In recent years, the precision associated with the fabrication of semiconductor and other various electronic devices has been increasing drastically. From the increases in precision, there has arisen the problem that particulate contaminants deposited on substrates, products, etc. during the fabrication processes of electronic devices cause defects in the products, thereby hindering the attempts to enhance the yield of the products. In the fabrication processes, therefore, cleaning techniques for satisfactory and efficient removal of the particulate contaminants deposited on the products are of extreme importance. The present invention relates to a cleaning apparatus for flat substrates, such as semiconductor wafers, glass substrates, magnetic disk substrates, magnetic head substrates, etc. which need high grades of cleaning.
One of the conventional equipments for cleaning of electronic parts and the like is described in Saishin Kyoryoku Choompa Gijutsu (Modern High-Intensity Ultrasonic Technology), edited by Eiji Mori, published by Extensive Technologies Service Center, Inc. (1987), pp. 223-239, in which an ultrasonic transducer is attached to a bottom or side surface of a cleaning bath. More specifically, this cleaning equipment is an immersion-type ultrasonic cleaning equipment which, as shown in FIG. 7, comprises a cleaning bath 13 filled with a cleaning liquid 15, and an ultrasonic transducer attached to the bottom surface of the bath 13.
Another one of the conventional equipments applies ultrasonic vibration to a thin layer of water formed preliminarily on a substrate by spraying, as disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 54-56255 (1979). As a further one of the conventional cleaning equipments, there is known a water discharge type ultrasonic cleaning equipment as described in Japanese Patent Application Laid-Open (KOKAI) Nos. 48-35671 (1973), 50-60677 (1975) and 63-266831 (1988). In this equipment, as shown in FIG. 8, cleaning water 18 is discharged from a nozzle 16, and an ultrasonic wave generated from an ultrasonic transducer 17 disposed on the back of the nozzle 16 is propagated to the discharged cleaning water 18, thereby effecting cleaning of a material 19 to be cleaned. Yet another cleaning equipment has been known, as disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 60-114386 (1985), in which the material to be cleaned is passed through a cleaning liquid flowing down, and an ultrasonic wave is applied to the material to be cleaned, whereby contaminant particles (hereinafter referred to as "particulate contamination"), oil or other film-form contaminants, metallic salts and the like (all these contaminants will be hereinafter referred to genetically as "contaminants" or "contaminations") are removed from the material to be cleaned.
However, the cleaning equipment comprising an ultrasonic transducer attached to a bottom or side surface of a cleaning bath as shown in FIG. 7 has the problem that the contaminants once removed from the material to be treated into the cleaning liquid will stagnate in the vicinity of the material to be cleaned, since there is little flow of the cleaning liquid. As a result, recontamination of the material to be cleaned occurs under attracting forces such as static electric forces, intermolecular forces, etc. exerted by the material to be cleaned, so that the material cannot be cleaned assuredly.
Also, the cleaning equipment based on the application of an ultrasonic vibration to a thin film of water preliminarily formed on the substrate by spraying is incapable of satisfactory cleaning of the material to be cleaned, because the effect to flow away the contaminants is small, as in the above-mentioned equipment, and the contaminants tend to remain on the material to be cleaned.
In the cleaning equipment based on discharge of water under ultrasonic vibration, as shown in FIG. 8, the cleaning effect is low in other areas than the area in which the discharged cleaning liquid collides against the material to be cleaned. With this equipment, therefore, it is impossible to clean evenly the entire surface of the material to be cleaned, and it is impossible to achieve satisfactory cleaning.
In the conventional cleaning equipment in which the material to be cleaned is passed through a cleaning liquid flowing down and an ultrasonic wave is applied to the material, the flow velocity of the cleaning liquid is so low that the contaminants tend to remain on the material to be cleaned. Accordingly, this type of equipment also fails to accomplish satisfactory cleaning.